This invention relates generally to the art of non-impact ink jet printing, and more particularly to ink pressure and temperature monitoring and control techniques for ink jet print heads.
Ink jet heads of the asynchronous type are described in U.S. Pat. No. 3,946,398 -- Kyser, et al. (1976), and co-pending patent applications Ser. No. 489,985, filed July 19, 1974, Ser. No. 694,064, filed June 7, 1976 and Ser. No. 807,219, filed June 16, 1977, all assigned to the assignee of the present application. In such an ink jet head, a piezoelectric crystal is associated with an ink jet chamber in a manner that when the crystal is supplied a high voltage pulse it rapidly reduces the volume of the ink jet chamber, resulting in ejecting a droplet of ink from an orifice with sufficient velocity for it to travel to a recording medium. One such droplet forms a small portion of a character to be printed. A plurality, such as seven or nine, of such chambers are preferably constructed as a single print head that is mechanically swept line-by-line across a recording medium upon which the printing is taking place. At each column of the printing line, the appropriate number of the independently controllable ink jet chambers are fired by pulsing their respective piezoelectric crystals to eject ink drops therefrom.
As described in the aforementioned co-pending patent applications, such a multiple channel print head preferably includes a common ink pulse chamber from which ink is supplied to each of the individual chambers. A source of ink is supplied under pressure to the pulse chamber through an electrically controlled valve. The pressure of the ink within the pulse chamber is monitored by a strain gauge connected to a mechanical beam that moves in direct porportion to changes of pressure within the pulse chamber. A second strain gauge is passively attached to a surface of the head. The two strain gauges are electrically connected in a bridge arrangement so that temperature variations do not significantly affect the pressure reading obtained. A signal proportional to the ink pressure is then utilized to open the ink supply valve when the pressure goes below a predetermined threshold and, conversely, to close the valve when the pressure goes above a certain threshold. In this way, the ink supply pressure to the individual channels is maintained within limits which aids in keeping the channels operating properly to eject droplets of ink when called upon to do so.
It is a principle object of the present invention to provide an improved ink pulse chamber pressure sensing circuitry, particularly circuitry that is less sensitive to extraneous induced noise.
It is also an important object of the present invention to provide a temperature sensing and control system for the print head in order that its temperature variation is maintained within a predetermined range, thereby to optimize the operation of the print head and improve its reliability.